<!doctype html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport"
          content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        body{
            background-color: red;
        }
    </style>
</head>
<body>
大页面
</body>
<script>
    window.onload = window.onresize = function (){
        if(window.innerWidth <= 800){
            //跳转到小页面中
            location.href = "./smallPage.html"
        }
    }
</script>
</html>